(CBS/AP) NEW YORK - Credit rating agency Egan-Jones is downgrading its rating on U.S. debt to AA- from AA, citing Federal Reserve plans to try to stimulate the economy.

The firm said the Fed's plans to buy mortgage bonds will likely hurt the economy more than help it. Egan-Jones said the plan will reduce the value of the dollar and raise the price of oil and other commodities, hurting businesses and consumers.

"Up, up and away -- the Fed's QE3 will stoke the stock market and commodity prices, but in our opinion will hurt the U.S. economy and, by extension, credit quality," Egan-Jones said in a report Friday, alluding to a third round of quantitative easing announced by the central bank Thursday. "Issuing additional currency and depressing interest rates via the purchasing of [mortgage-backed securities] does little to raise the real GDP of the U.S., but does reduce the value of the dollar... and in turn increases the cost of commodities (see the recent rise in the prices of energy, gold and other commodities)."

Egan-Jones said that, since 2006, U.S. debt as a ratio of GDP had grown to 104 percent, from 66 percent. By comparison, Spain's debt-to-GDP ratio of 68.5 percent, the firm added.

It is the second downgrade of the U.S. by the firm, a much smaller but well-known competitor to the big three rating
agencies: Moody's Investors Service (MCO), Fitch Ratings and Standard &
Poor's. In April, Egan-Jones downgraded the U.S. to AA from AA+. The company stripped the U.S. of a top AAA rating in July 2011.

On Thursday, the Fed said it would buy $40 billion of mortgage bonds a month to help the economic recovery.

See CBS News correspondent Rebecca Jarvis discuss the possible impact of the Fed's latest effort to stimulate the U.S. economy.